Pentachloronitrobenzene is a commercially important fungicide applied to soil and seeds in order to control various plant diseases, specifically those caused by botrytis, fusarium, rhizoctonia and anthracnase.
There are two broad industrial processes for the manufacture of PCNB. The first of these commercial processes is the nitration of pentachlorobenzene with nitric acid in sulfuric acid, as described by Breaux in U.S. Pat. No. 4,026,955. This process has also been described elsewhere, for example, U.S. Pat. Nos. 4,147,732; 4,138,438; and 4,057,590. The other, more important process for the production of PCNB involves chlorination of nitrobenzene in chlorosulfonic acid using iodine as a catalyst, as described, for example, by Thurston in "Fiat Formal Report No. 949". A variation of this process is disclosed by Lojewski in U.S. Pat. No. 3,026,358. Here in an attempt to reduce the final HCB content, chloronitrobenzene is used as the starting material . The PCNB obtained with the two types of prior art commercial processes described above contains HCB in concentrations above 0.1% by weight, even when mild reaction conditions that severely affect productivity are used.
Studies have demonstrated that HCB is an animal carcinogen, making the presence of HCB in fungicides undesirable. As a result, on Apr. 23, 1988 the Office of Pesticides and Toxic Substances of the Environmental Protection Agency set the environmentally acceptable standard level of HCB in PCNB at 0.1% by weight or less. Thus, in order to comply with this standard, commercial manufacturers have had to implement new technologies to reduce the HCB level in PCNB to 0.1% by weight or less.
Several processes for manufacturing PCNB having a reduced HCB content have been patented in recent years. These processes included the conversion of HCB to PCNB using pentaclorothiophenol (U.S. Pat. Nos. 4,454,362 and 4,461,918) and using pentachlorobenzonitrile (U.S. Pat. No. 3,984,487). These two routes represent significant advances in the production of relatively pure PCNB, however they have the disadvantage of using HCB as a starting material.
Another method for removing HCB from PCNB involves distillation, as described by Cazares in U.S. Pat. No. 4,842,696. This process has the disadvantage of requiring that the PCNB be melted prior to distillation and requires high temperatures and/or reduced pressures to effect the distillation. Due to the high temperatures, it is particularly critical that the crude PCNB have no basic impurities present in order to prevent the formation of dioxins, which are of extreme toxicological concern. This process has the further disadvantage of requiring solidification of the melt of PCNB after removal of the HCB.